-----按天統計
select to_char(t.hiredate, 'yyyy/mm/dd') 日期, count(1) 數量
	from EMP t
 where t.hiredate >= to_date('1980/1/1', 'yyyy/mm/dd')
	 and t.hiredate <= to_date('2017/1/31', 'yyyy/mm/dd')
 group by to_char(t.hiredate, 'yyyy/mm/dd')
 order by 日期;

---按月統計
select to_char(t.hiredate, 'yyyy/mm') 日期, count(1) 數量
	from EMP t
 where t.hiredate >= to_date('1980/1/1', 'yyyy/mm/dd')
	 and t.hiredate <= to_date('2017/12/31', 'yyyy/mm/dd')
 group by to_char(t.hiredate, 'yyyy/mm')
 order by 日期;

-----按年統計
select to_char(t.hiredate, 'yyyy') 日期, count(1) 數量
	from EMP t
 where t.hiredate >= to_date('1980/1/1', 'yyyy/mm/dd')
	 and t.hiredate <= to_date('2017/12/31', 'yyyy/mm/dd')
 group by to_char(t.hiredate, 'yyyy')
 order by 日期;
 
 -----按季度統計
select to_char(t.hiredate, 'q') 日期, count(1) 數量
	from EMP t
 where t.hiredate >= to_date('1980/1/1', 'yyyy/mm/dd')
	 and t.hiredate <= to_date('2017/12/31', 'yyyy/mm/dd')
 group by to_char(t.hiredate, 'q')
 order by 日期;
 
  -----按周統計
select to_char(t.hiredate, 'iw') 日期, count(1) 數量
	from EMP t
 where t.hiredate >= to_date('1980/1/1', 'yyyy/mm/dd')
	 and t.hiredate <= to_date('2017/12/31', 'yyyy/mm/dd')
 group by to_char(t.hiredate, 'iw')
 order by 日期;